Pressure filters of the plate and frame type have been extensively used and described in the prior art. These filters, described in further detail in the "Background of Invention" discussion of my co-pending application Ser. No. 739,755, generally comprise one or more sealed but separable chambers each of which contains one or more filter plates or leaves through which contaminated liquid is forced under pressure. Where multiple chambers are employed, the separation of the chambers and draining of the plates to allow the solids to be removed as a dry cake can be laborious, time consuming and expensive. Applicant's prior patents, such as U.S. Pat. Nos. 2,867,324, 2,867,325 and 2,867,326, improved upon the basic plate and frame type of filter by utilizing a simplified but completely automated single plate type of filter, permitting filtration and dry sludge removal without any manual attention.
One limitation on this type of pressure filter is the need for a strong supporting structure capable of withstanding the substantial internal forces generated by the hydraulic pressures. For example, a 10 psi internal hydraulic pressure requires 1440 pounds of external reaction force for every square foot of filter area. Since most of these filters are used on fairly porous suspended solids such as those generated from machining operations, the low hydraulic pressures which these machines can withstand are quite adequate for filtration. The sealing requirements and reaction forces generally limit this type of filter to 10 to 12 psi internal hydraulic pressure and 100 square feet of filter area.
Subsequent improvements by applicant included the use of expandable or inflatable seals such as shown in U.S. Pat. No. 3,306,458 and the use of a movable seal with stationary shells, such as shown in U.S. Pat. No. 3,333,693. A variety of configurations has been developed in an attempt to increase the flow rates and contaminant concentration capacities of the pressurized filters, including multiple separable filter shells, automated dirt discharge and combinations of permanent and movable filter belts. Examples of such systems are shown in applicant's further U.S. Pat. Nos. 3,335,862, 3,478,879 and 3,497,063.
Nevertheless, these prior pressurized filters did not have the capacity to handle the high filtration pressures required for applications such as dewatering of pulverized coal slurries after they have been conveyed through pipelines. In such an application, the high concentration of very fine suspended solids, such as 25-40% solids, accumulates rapidly with flow rates of 0.25 to 2 gallons per minute per square foot, quickly plugging up the filter cake pores, thus substantially reducing the flow rate. Only considerably higher pressures, such as 50 to 100 psi, can significantly improve flow rates and filter through-put capacity in such applications. Because the quantities of sludge involved in such systems may reach many tons per hour; it is desirable to develop filter systems having maximum filter area and the structural strength to resist high internal hydraulic pressures.
Applicant's above referred to co-pending application disclosed an improved mechanical locking system capable of accommodating hydraulic pressures of 100 to 250 psi in a two-shell pressurized filter system.
In spite of the above described advances in the art, substantial practical limitations remained to the significant increase in effective filter area. For example, increasing the number of vertically superimposed shells, to increase the amount of filter area for a given floor area, entails substantial problems and complexities in handling the multiple filter belts. One proposed solution to the problem has been utilized in the Synder type filter, wherein one pulling mechanism has been used to pull all of the disposable filter paper belts from the separated shells as required for belt indexing and/or cleaning. However, because of the weight of the sludge-laden belts and frictional forces involved, such a system as a practical matter has been limited to relatively small areas per shell and light dirt loads.
In addition to the above-described frictional forces, there are other problems involved in the handling of multiple rolls of filter belts. Because it is necessary in a multiple shell system for the shells to move vertically during separation for belt indexing (unless expandable or otherwise movable seals are used), the associated supply and discharge rolls for the belts preferably must also move vertically with the shells. This in turn would require elaborate roll-driving systems, such as separate motors, for each pair of supply and discharge rolls, because the vertically changing center-to-center distance between the axes of the rolls does not facilitate driving one level of rolls from an adjacent level. This requirement obviously introduces a substantial cost penalty where multiple shells are utilized.
Accordingly, it is the principal object of the present invention to provide an improved pressure type filter capable of handling high internal hydraulic pressures, and having multiple vertically superimposed shells each of substantially increased area relative to those of the prior art and a filter that can discharge large dirt loads.